The two matrons at the start each have a damage aura which causes any mobs near them to attack twice as hard and fast. CC them or tank them far away from eachother. When they howl, get ready to remove a corruption. Any corruptions left on by the time they howl again will cause massive heals which cannot be dps'd through. If they do heal, turtle up and wait it out. EVENTUALLY, if you remove your wounds fast enough and out survive them, the Matrons will run out of power and no longer be able to heal. The Warg boss is simple as long as your remove wounds quickly. Captains revealing mark is the most overpowered skill ever in this fight =P
The next point are the Swarming Bats. They should ALWAYS be your priority target. Use target assists, focus fire and get them down fast. Once they are down, all their pets will simply disappear.
Task-Mistresses. Count your group down everytime you engage them. Make sure the area around is clear of mobs so you only have to deal with the mistress. Note: you can move right into melee range before engaging, they are "threateners". I recommend making use of this to save time. Have someone keep an eye on the the timer for her buff. Get them to call it out when it gets to 10s. At this point, unless the mistress is within a few hits of dying, the person with agro should begin to kite whilst everyone else chews her down. Also, as always, remove wounds as quickly as possible.
Defilers are fairly simple. They cast a corruption on themselves which reduces their incoming damage by ~80%. If you are in a rush, then clearly leaving this on is going to hurt you in the long run. They also cast an interruptable skill which has an AoE power drain. Not the end of the world but interrupting is advised. The first time you encounter a defiler is before the second chest. You may have noticed, or you should have noticed, a task mistress walking up to a mirror and "corrupting" it by summoning the defiler. This is an important point. The defiler is what causes the mirror to stop reflecting and the mistress is what spawns the defiler. Knowing this, next time you run into this room, run STRAIGHT to the back and tag the mistress ASAP, i.e. before she gets to the mirror. Hey presto, no defiler.
The same applies for the bottom puzzle. Get down as fast as you can, tag the mistress and kill her before she gets to the mirrors. Then, have two people run around and kill any defilers that are up (hopefully there aren't any) and set mirrors in the correct positions. The remaining person should stay in the middle platform and wait for mistresses to spawn. When she does, the person should go close and call it out. The mistress will stop to "threaten", giving time for the other two to get near (possibly dragging defilers with them but hopefully not). Focus fire the mistress, remove wounds fast, start kiting at 10s if necessary. Once all the mirrors are set and the web opens up, no more mistresses will spawn.
Take a minute to select merrevail around the edge and target mark them. There are 4 types and they sit symmetrically such that any four in a row are bound to be the four different types. Once you have target marked one of each type, pop scrolls, tokens, food and jump down to face the boss. My preference is to drag the boss in the first phase over to the OPPOSITE side of where the 4 target marked mobs are. That way, when Ergoth goes off for a breather, the target marked mobs take longer to reach me and the others get to me fast, allowing me to let loose one big AoE and either kill, or nearly kill a lot of the unmarked ones. The next thing is to tab through and kill all of the UNMARKED mobs as fast as possible. So far the best I have done is 10 out of 12 in the first wave. If you accidently kill any of the marked mobs, you lose hardmode and so you might as well kill them all. Care with AoE's is critical here, if in doubt, don't use any at all.